This invention relates to a process to reduce silica concentration of solutions already supersaturated in silica and more specifically of Bayer liquors obtained from the digestion of bauxite in a caustic liquor.
In the Bayer process for producing alumina from bauxite, the bauxite containing aluminium hydroxides is contacted with solutions containing caustic soda to dissolve the aluminium hydroxides and some of the silica minerals, such as kaolinite, while leaving most of the remaining constituents of the bauxite essentially unattached in the solid form. The silica which is dissolved by the caustic soda solution forms a soluble sodium silicate. This reacts relatively slowly with the sodium aluminate in solution to form complex hydrated sodium aluminium silicates, known collectively as "desilication products". These desilication products, which include Bayer sodalite, are of low solubility in the resulting sodium aluminate-caustic solutions and eventually precipitate out of the solution.
Silica is a highly undesirable impurity in the Bayer liquor for two reasons: (1) when present in a super-saturated aluminate liquor it will coprecipitate with the alumina downstream and will contaminate the product, (2) when present in the Bayer liquor at a concentration above its solubility, it tends to precipitate mainly on the surface of heat exchangers, thus greatly reducing the heat transfer coefficient.
In order to keep the silica concentration in the liquor at an acceptable level, most current Bayer plants maintain the digestion conditions for the extraction of alumina from the bauxite for a time sufficient that the extracted aluminium hydroxides and the dissolved silica have time to form the desilication products prior to the separation of the unattached solids. This process may take up to an hour. In certain cases, the digestion step may also be preceded by a pre-desilication step performed at lower temperature, which is aimed at dissolving some silica in order to favour the subsequent silica precipitation reaction. Because of the longer residence time required at digestion conditions (high temperature and pressure) to allow the silica precipitation reaction to occur, premature alumina crystallization will occur and hence reduced productivity. In addition, a longer residence time means that larger size pressure vessels are required and energy requirements are higher.
U.S. Pat. No. 5,118,484 describes a process for removing dissolved silica from Bayer process solutions which comprises contacting the silica-containing solution with porous agglomerates of a material containing Bayer process desilication products bonded together by a polymeric resin. These porous agglomerates are contained in a column, with the Bayer process solution being passed through the column. The agglomerates provide a large surface area upon which is precipitated the desilication product formed by reaction between the dissolved silica and the alumina and sodium present in the Bayer process solution.
Another process in which the desilication stage is separate from the digestion stage is described in U.S. Pat. No. 4,426,363. In that case, the desilication is carried out prior to the step of separating the slurry into caustic aluminate solution and insoluble mud. That procedure has the disadvantage of transporting a very large amount of solids of which only a small portion is effective in acting as a seed surface for the desilication process.
In U.S. Pat. No. 5,545,384 there is described a process where the bauxite-liquor slurry, after being digested at a temperature between 120 to 160.degree. C., is separated and the extract solution is sent to a desilication step which takes place after the separation of the mud. That patent recommends the use of seeds of the solid silicate substance comprising sodalite or zeolite as major components and it also recommends that these seeds should have an average particle size of between 1 .mu.m and 30 .mu.m.
It is the object of the present invention to provide a further simplified desilication procedure which can be utilized independently from the digestion stage.